Wind energy as been used for centuries to sail ships, pump water, and grind corn and other grain. In the late 1800's, wind turbines for generating electricity were developed. As fossil fuels became more economical, interest in windmills and wind turbines declined substantially, until the Arab oil embargo in 1973 generated an increased interest in alternative energy sources, including wind energy.
Today, the most common wind turbines for generating electricity are horizontal axis turbines, also known as HAWTs.
Despite substantial growth in the commercialization of HAWTs beginning in the 1990's, one major disadvantage of the HAWT is the need to shut the turbine down when wind speeds exceed a particular value, known as the cutoff speed, for safety purposes, particularly relating to the large blades of the HAWT. The typical cutoff speed ranges between 20-25 meters per second. This limitation of HAWTs makes them unsuitable for storm prone areas.
There are two general classifications or categories of vertical axis wind turbines, VAWTs, namely, drag and lift based designs. In a drag based VAWT, the blade speed rarely exceeds the wind speed. The drag is generally described as the pressure or the thrust on the turbine blades created by the wind as the blades pass through the wind. Drag based VAWTs have relatively higher starting torque and less rotational speed than lift based VAWTs. Drag based VAWTs are generally unsuitable for producing electricity due to the low speed, but are useful for grain grinding, pumping water, and other high torque propulsion applications. One major advantage of the drag based VAWT is the ability to self start.
Lift based VAWTs rely on aerodynamics. Lift based VAWTs have a low starting torque, but high rotational speed, so as to be suitable for generating electricity, but normally are not self-starting.
VAWTs have had limited commercial success, primarily by the manufacture of the machine by Flow Wind, which went bankrupt in 1997. Such VAWTs were inferior and less efficient than HAWTs, and therefore have been substantially abandoned in the marketplace. Variations of the Darrieus VAWT include the giromill, which has straight vertical blades, and a cycloturbine, which uses a wind vane to mechanically orient the blade relative to the wind. Other types of VAWTs utilize ducts and/or vortex generation towers, with shrouds or diffusers to deflect the horizontal wind stream to a vertical stream with increased speed.
HAWTs and VAWTs have advantages and disadvantages as compared to one another. A VAWT is traditionally omni-directional, in that it does not need to be turned into the wind as the wind changes direction, as with a HAWT. Thus, a VAWT does not require the yaw mechanism required by a conventional upwind HAWT for proper orientation into the wind. HAWTs also are normally mounted on a high tower to take advantage of higher wind speeds at increased elevations, as compared to wind speed at the ground level. VAWTs are normally placed near the ground, which allow the heavy gear box, generator and control mechanism to be positioned on the ground, thereby permitting easier erection and accessibility for maintenance. The height of HAWTs also create a more pronounced visual impact on the surroundings, while the lower height VAWTs are considered to be more friendly to the visual environment. The blades of a VAWT are not subjected to stresses due to vertical wind velocity gradients and gravitational forces during rotation. In comparison, HAWT blades, which may be 100 feet long, pass through a vertical wind gradient with increased wind speeds atop of the blade path, as compared to the bottom of the blade path. HAWT blades also are subjected to loading due to gravitational forces as the blades move upward and downward in their circular path. Thus, the HAWT blades and blade roots are subject to low cyclic fatigue, which has a bearing on cost and longevity. VAWT blades normally are less expensive and have a longer life expectancy than HAWT blades. HAWT blades are also subject to cantilever loads resulting in bending moments at the blade roots. In comparison, VAWT blades balance centrifugal loads by pre-tension forces in the blades, thereby avoiding bending moments, with the turbine shaft carrying axial and torsion loads only.
Since VAWTs are normally mounted closer to the ground than HAWTs, VAWTs are subject to less wind speed and more turbulence caused by ground sheer. Therefore, for a given rotor size and weight, the VAWT output power is less than that of a HAWT. VAWTs also normally required guy cables for support, which increases the footprint size. The torque and power output of a VAWT fluctuates in a cyclic manner during each revolution as the blades move into and out of the wind, whereas the torque and power output of a HAWT is relatively steady. The cyclic torque variations in the VAWT creates natural vibration frequencies which must be avoided to prevent resonance damage to the motor. HAWTs generally have no vibration problems.
Giromills, which never progressed beyond the research stages, and cycloturbines which had limited commercial success, both were normally placed upon high towers, as compared to the Darrieus VAWT which did not include a high tower. In a cycloturbine, the blades are mounted so that they rotate around their vertical axis, such that the blades can be pitched for an angle of attack relative to the wind. The cycloturbine generates the relatively constant torque. The cycloturbine is capable of self starting by pitching the one blade flat to the wind to generate drag and start the turbine spinning at a low speed. However, the blade pitching mechanism is relatively complex and generally heavy.
Accordingly, a primary objective of the present invention is the provision of an improved wind turbine for generating electricity and/or providing mechanical energy.
Another objective of the present invention is the provision of a windmill having multiple blades which rotate upon a blade axis as the turbine rotates about a turbine axis.
Another objective of the present invention is the provision of a wind turbine which can be oriented horizontally or vertically.
Still another objective of the present invention is the provision of a wind turbine having a simple blade pitch adjustment mechanism.
A further objective of the present invention is the provision of a windmill having a fan tail to automatically direct or control the orientation of the blades relative to the wind as the turbine rotates.
Another objective of the present invention is the provision of a windmill having a plurality of rotatable blades and a plurality of blade directors, with one director being associated with each blade, so as to orient the blade relative to the wind.
A further objective of the present invention is the provision of a windmill having a frame which rotates in one direction and blades which rotate in the opposite direction.
Another objective of the present invention is the provision of a windmill which is economical to manufacture, and efficient and durable in use.
These and other objectives will become apparent from the following description of the invention.